The invention relates to a piezoelectric flexural transducer possessing an elongated support body, which on at least one longitudinal side is provided with a piezoelectric multi-layer body having a plurality of layers of piezoelectric material and intermediately placed electrodes. The invention furthermore relates to the use of such a piezoelectric flexural transducer.
Such a piezoelectric flexural transducer, which may also be described as a multi-layer flexural actuator, is disclosed for example in the patent publication WO 99/17383. This piezoelectric flexural transducer comprises an elongated, plate-like or, respectively, bar-like support body, which on its two oppositely placed longitudinal sides with a large area is provided respectively with a piezoelectric multi-layer body. Each piezoelectric multi-layer body is composed of a plurality of superposed piezoelectric material layers and furthermore sheet-like electrodes arranged between the piezoelectric material layers. An electrical conductor, which extends over the body, can be utilized to apply a control voltage causing a contraction in length of a piezoelectric multi-layer body, this resulting in a deflection of the piezoelectric flexural transducer perpendicularly to the longitudinal direction thereof.
For the operation of the piezoelectric flexural transducer recourse is as a rule had to a suitable electronic system, whose components ensure, for example, the conversion of an available low voltage current into the sufficiently high control voltage. In this respect multi-layer technology offers the advantage that the electrical energy necessary for the operation of the piezoelectric flexural transducer is available even at a substantially lower control voltage than in the case of a comparably dimensioned monolithic piezoelectric body. However, the electronic components do have a substantial requirement as regards necessary space and this hinders the miniaturization of devices, as for example valves, having the piezoelectric flexural transducer.
A further problematical issue with the known piezoelectric flexural transducers is the monitoring and/or feedback supply as regards the operational behavior. The provision of sensor technology suitable for this purpose is again something which is unfavorable as regards the overall size of the device fitted with the piezoelectric flexural transducer.
Accordingly one object of the present invention is to indicate measures, which render possible more compact dimensions of devices having a piezoelectric flexural transducer of the type initially mentioned. A further aim of the invention is as regards a suitable application of such a piezoelectric flexural transducer.
The first mentioned aim in connection with a piezoelectric flexural transducer of the type initially mentioned is attained since in the interior of the support body and/or between the support body and the piezoelectric multi-layer body and/or between adjacent piezoelectric material layers of the piezoelectric multi-layer body at least one component of an electronic and/or sensor system, which is utilized for the operation of the piezoelectric flexural transducer, is accommodated.
The invention is based on the recognition that the components, to be employed in conjunction with a multi-layer flexural transducer, of the electronic and/or sensor system necessary for operation may be so designed as regards their dimensions and their geometry, that an integration in the piezoelectric flexural transducer is possible and it is accordingly possible to dispense with electronic and/or sensor components, which would necessitate a larger amount of space for their installation. Thus the electronic and/or sensor components may for example be accommodated in the interior of the support body, although they can be also placed between the piezoelectric material layers of the piezoelectric multi-layer body or between the support body and the piezoelectric multi-layer body mounted on same. In connection with the sensor components this then opens up the possibility of placing same practically at that position, where an operation-relevant parameter is to be detected.
Thus for example in the case of safety-relevant valve applications reliable and exact feedback information on the actuator setting may be ensured.
It has been found to be particularly advantageous to provide the piezoelectric flexural transducer with a piezoelectric multi-layer body, in the case of which the layer thickness of the individual piezoelectric material layers is 25 xcexcm at the most, a thickness range between 14 xcexcm and 20 xcexcm being recommended, the most preferred layer thickness being 17 xcexcm. This results a low voltage multi-layer structure having extremely thin layers and accordingly a much reduced component drive voltage, which is substantially less than the state of the art component voltages in the range above 60 volts. Accordingly the structural complexity of the circuits necessary for voltage conversion is also reduced, this simplifying the integration in the piezoelectric flexural transducer in accordance with the invention even more. Moreover, there is the advantage that design standards applying for the high voltage range are no longer relevant and accordingly a greater variety of design possibilities is opened up for the placement of wiring and the design of the housing in the piezoelectric flexural transducer or, respectively, equipment fitted with it. Nevertheless sufficient electrical field strength is achieved in the piezoelectric material and, as having regard to the overall size, larger setting forces and larger displacements are possible.
Further advantageous developments of the invention are defined in the dependent claims.
At least one of the integrated electronic components may form part of a voltage converter circuit, a current limiting circuit or a charging and discharging circuit or directly itself constitute one such circuit.
At least one such sensor component may be constituted by a displacement sensor responsive to deflection or displacement of the flexural transducer, for example in the form of a strain gage strip (DMS), by one or more piezoelectric material layers or by inductive pickups and coils. A substantial advantage of a displacement sensor integrated in the support material or in the piezoelectric material lies in the possibility of use in the case of systematic follow-up of the actuator""s displacement for the compensation of the piezoelectric relaxation and drift effects. This renders possible control applications with long term stability. A further possibility of use is functional monitoring of the setting element.
Furthermore additional sensors of an desired type and structure may be integrated in the system, for example force, acceleration or temperature sensors.
The integration of one or more electronic and/or sensor components in the support body may be performed particularly simply, if the support body is designed in the form of a multi-layer body having at least two superposed support body layers, the layer planes of the support body being preferably parallel to those of the piezoelectric material layer and the respective components being able to be placed betwixt two respective neighboring layers of the support body.
In order to be able to provide the electrical conductors required for the control of the piezoelectric multi-layer body and furthermore for the operation of any sensor systems present with an optimum distribution on the support body, the electrical conductors present are preferably arranged in different component carrying planes of the support body, the mutual connection being produced by means of one or more metal lined holes or metal filled holes, so-called vias, which extend athwart the fitting planes. In the case of a support body produced using multi-layer body technology electrical conductors may be provided both between neighboring layers of the support body and also on the outer face of the outer support body layers, electrical conductors, which are located on either side of a support body layer, being able to be contacted by means of metal lined holes or metal filled holes, which merely extend through the respective support body layer.
This structure is more especially advantageous when the piezoelectric flexural transducer possesses a so-called trimorphous form, the support body being fitted with two piezoelectric bodies, which are set on two mutually remote longitudinal sides of the support body. In this case the two piezoelectric bodies are preferably designed in the form of piezoelectric multi-layer bodies of the above mentioned type in order to render possible a forced displacement of the piezoelectric flexural transducer in two mutually opposite directions on the basis of the above mentioned advantages. The design may be such that the trimorphous flexural transducer has a middle layer in the form of a fiber compositexe2x80x94in conjunction with flexible printed wiring layers as a contact means with the active piezoelectric material layers, with the sensor system and/or with the electrical circuitry. With a suitable selection of materials having suitable coefficients of thermal expansion a systematic reduction of thermal drift is obtainable. One example for this is the systematic incorporation of copper layers in the overall structure of the actuator. In addition the reduction in thermal drift the copper layers may also be utilized to set the specific mechanical properties such as displacement and setting force and furthermore for contacting the active piezoelectric material layers, the integrated sensors and/or the electronic circuitry.
Instead of a second active piezoelectric body it is furthermore an advantage to provide an adaptive body, made of a material having-essentially the same coefficient of thermal expansion as the piezoelectric multi-layer body. This adaptive body does not contribute to producing displacement forces, that is to say it is therefore a passive element, although it is responsible for ensuring that, in comparison with a piezoelectric flexural transducer also having a piezoelectric body on one side thereofxe2x80x94a so-called unimorphous typexe2x80x94it is possible have a symmetrical design at relatively low cost so that inherent thermal flexure can be reduced or completely excluded. Moreover, it is possible for the adaptive material itself to be a sensor or to be integrated in a sensor. The sensor may also be integrated in the adaptive part.
The initially mentioned second object of the invention intended for achieving certain tasks, is made possible by having the piezoelectric flexural transducer in the form of a setting member of a valve, more particularly in the pneumatics sector for the control of air flows.